Mass spectrometer magnetic analyzer having pole pieces composed of spaced paramagnetic laminae



Sept. 2, 1969 M R v 3,465,142

MAss SPECTROMETER MAGNETIC ANALYZER HAVING POLE I PIECES COMPOSED OFSPACE!) PARAMAGNETTC LAMINAE A Filed Aug. 10, .1966

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United States Patent 3,465,142 MASS SPECTROMETER MAGNETIC ANALYZERHAVING POLE PIECES COMPOSED OF SPACED PARAMAGNETIC LAMINAE Thomas OliverMerren, Mawdesley, England, assignor to Associated Electrical IndustriesLimited, London, England, a British company Filed Aug. 10, 1966, Ser.No. 571,463 Int. Cl. H01j 39/36; B01d 59/48 US. Cl. 250-413 9 ClaimsABSTRACT OF THE DISCLOSURE In a mass spectrometer apparatus including amagnetic analyzer establishing a path of ion travel from an ion sourceto a detector the magnetic pole pieces are composed of a plurality ofspaced paramagnetic laminae having insulating elements therebetween. Thelaminae are stacked along an axis paalleling the beam path with thelaminae positioned transversely with respect to the beam path.

This invention relates to improvements in mass spectrometers and massspectrometry, and an object of the invention is the provision ofapparatus which improves the resolution of a mass spectrometer at highscanning speeds.

According to the present invention a mass spectrometer includes meansadapted to produce a beam of ions of a material to be analyzed, amagnetic analyzer in which a magnet produces a magnetic field which istransverse to the ion beam and differentially deflects the ions of thatbeam in a direction which is transverse to the beam in accordance withtheir mass/charge ratios, means for producing scanning of themass/charge ratio spectrum of the ion beam, and output means providingan indication and/ or record of the scanned part of the spectrum of thedeflected beam, wherein the magnet poles of the magnetic analyzer arebuilt up of discrete magnetic elements electrically insulated from eachother to inhibit eddy currents which would produce irregularities in themagnetic field, thereby to provide optimum focusing of the beam duringscanning and thus to improve the resolution in the output from theoutput means between the various peaks in that output.

Preferably the magnetic elements are in the form of laminae ararngedwith their planes parallel to the lines of force between the magnetpoles of the magnetic analyzer and transverse to the path of the ionbeam, but they could take the form of rods or bars arranged with theirlongitudinal axes parallel to the lines of force between the magnetpoles, or could take the form of regular shaped parts, such as cubes, oreven relatively small particles providing that the hysteresis loop ofthe resulting structure is not too broad.

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which:

FIGURE 1 is a diagrammatic sectional side elevation of a massspectrometer; and

FIGURE 2 is a perspective drawing of one magnet pole of the magneticanalyzer shown in FIGURE 1.

Referring first to FIGURE 1, the mass spectrometer includes an ionsource chamber 1 into which a specimen carrying probe 3 can be insertedand in which ions can be liberated from that specimen. An electrode 5 towhich an accelerating voltage is applied serves to repel these ions as abeam which passes first through an electrostatic analyzer 7 includingopposed conductive plates 7P between which a potential difference ismaintained, then through an intermediate slit 9 into a magnetic analyzer11. In the magnetic analyzer 11 an electromagnetic coil 11C establishesa strong magnetic field directed in a direction transverse to the pathof the ions, and since the ions are charged particles their paths willbe curved in the magnetic field. The deflected ions, or certain of themif different groups of ions are deflected to different degrees, passthrough an adjustable slit in a member 13 and are picked up by acollector electrode 15 associated with an electron multiplier 17.

Mass spectrometers as described above are well known in the art, and theoutput from the electron multiplier 17 is used, after amplification, toprovide a record of the ions passing through the adjustable slit inmember 13. The angular deflection of an ion in passing through themagnetic analyzer 11 will depend upon the accelerating voltage, sincethat determines the speed of the ions, the intensity of the field in theanalyzer 11, and the mass of the ion. One method of scanning a largerange of a mass spectrum is to maintain the voltages used in theelectrostatic analyzer 7 and on the accelerating electrode 5 constant,and to scan by varying the current used in the electromagnet coil of themagnetic analyzer 11. This progressively changes the deflections of allthe ions passing through the magnetic analyzer, so that the output fromthe electron multiplier 17 indicates the number of ions passing throughthe slitted member 13, and when presented on a cathode ray tube as thevertical deflection with a horizontal scanning speed corresponding tothe decay or growth of the magnetic field in the magnetic analyzer 11,the trace shows peaks where ions having such a mass number that they aredeflected to pass through the slitted member 13 are present.

It has been found that an improvement in the resolution can be obtainedif certain irregularities in the field of the magnetic analyzer whichare produced by eddy currents can be overcome. The eifect of eddycurrents induced within the magnet poles during the scan is to alter themean magnetic field through which the ions travel and thus alter thefocal length of the magnetic field of the magnetic analyzer. Thus, formaximum resolution, the magnet poles of the magnetic analyzer 11 arebuilt up of discrete magnetic elements which are electrically insulatedfrom each other to inhibit these eddy currents. Referring now to FIGURE2, which shows a preferred form of these magnetic elements, a yoke 31 ofthe magnetic analyzer has secured thereto an arcuate pole piece 33 builtup of laminae 35 of magnetically soft material which are electricallyinsulated from each other. Typically, the laminae 35 which are arrangedwith their planes substantially normal to the path of the ion beamindicated by dotted line 37, have a length x of four inches, :1 width yof at least one inch and are about 0.014 inch in thickness. The laminaeare separated from each other by a layer of insulating material 0.001 ofan inch thick. In order to closely approximate to the arcuate shape ofthe pole piece 33, using substantially flat lamina, wedge shapedmagnetic members 39 may interspace groups of the laminae 35. Themagnetic scanning is produced by a control unit 51 arranged toprogressively reduce the energizing current applied to the electromagnetcoil 11C. The laminae inhibit eddy currents which would produceirregularities in the magnetic field during the scan and thereby improveresolution in the output from the output device between various peaks inthe output.

The maximum speed at which the magnetic field can be scanned may also belimited by eddy currents flowing in the yoke 31 of the magnetic analyzerand these eddy currents may be inhibited by constructing the yoke 31from laminae 41 of magnetically soft material electrically insulatedfrom each other.

The invention is also applicable to mass spectrometers which do not havean electrostatic analyzer.

What I claim is:

1. A mass spectrometer comprising:

(a) an ion source for generating a beam of ions;

(b) a detector for receiving, measuring, and recording ions from saidsource;

(c) structure including a magnetic analyzer establishing a path of iontravel from the source to the detector; and

((1) said analyzer including a paramagnetic assembly having a pluralityof paramagnetic elements, adjacent ones of said elements being spaced toprovide insulating gaps therebetween with each of said gaps havingelongated dimensions in a plane transverse to said path and a shortdimension generally parallel to said path such that the elements arearranged to parallel magnetic lines of force of the analyzer.

2. A mass spectrometer as claimed in claim 1, wherein the paramagneticelements comprise rods or bars arranged with their longitudinal axesparallel to the lines of force between the magnet poles of the magneticanalyzer.

3. A mass spectrometer as claimed in claim 1, wherein the paramagneticelements comprise regular shaped parts.

4. A mass spectrometer as claimed in claim 1, wherein the paramagneticelements comprise relatively small particles.

5. A mass spectrometer as claimed in claim 1, wherein the analyzerincludes a yoke of the magnetic analyzer built up of discreteparamagnetic elements electrically insulated from each other to inhibiteddy currents which would produce irregularities in the magnetic field.

6. A mass spectrometer as claimed in claim 5, wherein the paramagneticelements forming the yoke comprise laminae.

7. A mass spectrometer comprising:

(a) an ion source for generating a beam of ions;

(b) a detector for receiving, measuring, and recording ions from saidsource;

(c) structure including a magnetic analyzer establishing a path of iontravel from the source to the detector;

((1) said structure including a laminated pole piece composed of aplurality of spaced paramagnetic laminae and insulating elementstherebetween; and

(e) said laminae being stacked along an axis paralleling said beam pathwith the laminae and elements positioned transversely with respect tosaid path.

8. A mass spectrometer as claimed in claim 7 wherein the laminae includewedge-shaped paramagnetic members interspaced with groups ofsubstantially flat members to provide magnet poles of arcuate shape.

9. The mass spectrometer of claim 7 wherein each of said laminae isrectangular in cross section in a plane of cross section transverse tosaid ion path.

References Cited UNITED STATES PATENTS 2,541,656 2/1951 Long. 2,279,0144/1942 Sawyer 336-219 X 3,226,587 12/1965 Chin 335210 X RALPH G. NlLSON,Primary Examiner A. L. BIRCH, Assistant Examiner U.S. Cl. X.R.

